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SUMMARYThe laminar flow in the small bronchial tubes is quite complex due to the presence of vortex-dominated, secondary flows. In this paper, we report the results of a numerical investigation of the simultaneous effects of asymmetric and non-planar branching on the primary and secondary flows in the small bronchial tubes, i.e. generations 6-12. We simulate steady-state inspiratory flow at a Reynolds number of 1000 in three-generation, asymmetric planar and non-planar bronchial tube models. The non-planar model was defined by applying a 90 • out-of-plane rotation to the third-generation branches. A detailed mesh refinement study was performed in order to demonstrate mesh independence. Significant differences were observed between flows in the planar and non-planar models. An uneven mass flow distribution was observed in the non-planar model in contrast to the evenly distributed mass flow in the planar model. The secondary flows created symmetric vortex patterns in the planar model, whereas vortex symmetry was lost in the non-planar model. These results illustrate the importance of incorporating asymmetry in addition to non-planarity in the geometric models.

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The laminar flow in the small bronchial tubes is quite complex because of the presence of vortex-dominated, secondary flows. Factors contributing to this complexity are the unsteady nature of the inhale-exhale breathing cycle and the geometrical characteristics of the bronchial tubes. To investigate unsteady effects on flows and particle transport, unsteady inhalation flows at a 30-respiration-per-minute frequency, corresponding to a moderate activity level, were simulated for a three-generation, asymmetric, planar bronchial tube model. Ten-micron diameter water droplets were introduced at the inlet at different times during inhalation to develop particle destination maps. The differences in the flow fields and destination maps obtained at the unsteady peak flow and the comparable steady-state inflow condition were minimal. However, particles released at equivalent instantaneous off-peak inflow conditions produced different destination maps. The differences were attributed to the temporal variations of the fluid velocities and history effects.